1. Field of the Invention
The present invention relates to a false call protecting apparatus for use with a telephone exchange system and a false call protecting method, in particular, to a false call protecting apparatus for use with a telephone exchange system that has an exchange that causes an international call to be charged on the terminating side and a false call protecting method thereof.
2. Description of the Related Art
Conventionally, a credit card call service has been widely used. In the credit card call service, when the user dials a toll-free number with his or her credit card number to make an international call, the international call is charged on the terminating exchange side. Against such a service, a false call with a deception of a charging system of the toll-free service has thrived.
Conventionally, to protect such a false call, an illegal signal is detected from a signal sequence and the call thereof is disconnected. However, due to a delicate difference between line signal receiving characteristics of exchanges, the illegal signal cannot be sometimes recognized. Thus, a false call cannot be completely protected and thereby it has thrived.
In conventional telephone exchange systems that have a line signal receiving function according to International Telecommunication Union Telecommunication Standard ITU-T recommendations, it is difficult to completely detect an illegal signal. This invention is especially directed to Q140-Q164 of the ITU-T No. 5 standards issued November 1988 and March 1993 and dealing with a credit card and especially to Q140-Q144, Q146, Q151, Q152, Q153 and Q154. These sections will be identified in whole or in part in the appended claims by the phrase "at least one version of . . . ITU-T No. 5 Signal System".
As countermeasures against a false call on a terminating international station side, after it receives a CLF signal from an originating station side, the transmission time of a release guard (RLG) as an answer signal thereof may be increased, However, unless all terminating international stations take such countermeasures, false calls cannot be completely protected Thus, such countermeasures should be taken on the originating international station side.
Consequently, originating international stations of such a telephone exchange system do not have a means for detecting a false signal received from a telephone set of a false originating side. Thus, since the telephone set on the false originating side sends an false signal to an terminating international station, the security cannot be maintained.